The present invention relates to variant thioesterases and their use in plants, e.g., to increase enzymatic activity and to promote increased production of mid-chain length fatty acids (e.g., 8 to 14 carbons) and at desired ratios. Further disclosed herein are methods of manufacturing renewable chemicals through the manufacture of novel triglyceride oils followed by chemical modification of the oils. Oils containing fatty acid chain lengths of C8, C10, C12 or C14 are also disclosed and are useful as feedstocks in the methods described herein.